dmc wrote:

Question one: How likely am I to be able to drill a hole in a rock. Something
like 15mm I guess. I have a decent SDS drill is this likely to just smash the
thing up? Should I be looking for a non-hammer solution (diamond?).

Rather depends what sort of rock it is how hard it might be,
possibly best to try drilling with the hammer off and to
start off with a 6mm drill bit.

The other thing is that this "rock" had what the seller called "a special
halogen bulb" in the hole so that the water bubbled up over the bulb.
According to my dad this looked like a small bulb with heatshrink on with
just a couple of wires sticking out of the end. These were 25quid each...

Question two: Any ideas what this "special halogen bulb" might have been?
Maybe a super bright white LED would work? Dad wasn't convinced that it
was a real bulb at all.

An LED has the advantage of never needing replacing. Possibly more than
one arranged in a circle with a gap in the middle. Something like 6
LEDs, 6 resistors, some wire heatshrink sleaving and expoxy resin should
cost a lot less than 25 quid.

I suppose one answer to drilling the hole is to make the rock - anyone
got any recipies for half realistic rock mixes? :-)

Portland cement makes a reasonable subsitute for Portland
limestone